Data communications between electronic devices such as integrated circuits in a system are in general constrained by the behavior of the interconnecting transport medium, such as wires, printed circuit traces, or optical fibers. Transmission line effects including attenuation, signal reflections, and frequency-dependent propagation characteristics distort transmitted signals, requiring corrective solutions to be applied. Linear circuit corrective measures include amplification of received signals, and frequency-domain signal correction using as one example Continuous-Time Linear Equalization (CTLE).
Data-dependent equalization is also well known in the art. Generally, these time-domain-oriented equalization methods focus on compensating for the effects of inter-symbol-interference or ISI on the received signal. Such ISI is caused by the residual electrical effects of a previously transmitted signal persisting on the communications transmission medium, so as to affect the amplitude or timing of the current symbol interval. As one example, a transmission line medium having one or more impedance anomalies may introduce signal reflections. Thus, a transmitted signal will propagate over the medium and be partially reflected by one or more such anomalies, with such reflections appearing at the receiver at a later time in superposition with signals propagating directly.
Digital corrective measures can be applied at the transmitter, as one example using pre-equalization with Finite Impulse Response (FIR) filtering, and at the receiver using methods including Feed-Forward Equalization (FFE) and Decision Feedback Equalization (DFE).
Decision Feedback Equalization is performed by maintaining a history of previously-received data values at the receiver, which are processed by a transmission line model to predict the expected influence each of the historical data values would have on the present receive signal. Such a transmission line model may be pre-calculated, derived by measurement, or generated heuristically, and may encompass the effects of one or more than one previous data interval. The predicted compensation for the influence of these one or more previous data intervals is collectively called the DFE correction, which may be explicitly combined with the received data signal prior to receive sampling of the resulting corrected signal, or implicitly combined by using the DFE correction to modify the reference level to which the received data signal is compared in the receive data sampler or comparator.